Supermarket giants Coles and Woolworths are under pressure to give the money raised from reusable plastic bags back to dairy farmers or scrap their cheap milk for good.

From July 1, supermarkets will be banned from offering single-use plastic bags in all states and territories bar NSW.

Barnaby Joyce is leading a National Party outcry and threatening a political firestorm if the retailers don't give the money raised from customers buying new 15c reusable bags back to Queensland farmers.

"How you can justify 15 cents for a plastic bag and a dollar a litre for milk, we have no idea," Mr Joyce said.